Waveguides in photonic integrated circuits have been proposed in a variety of forms but most typically in the form of a doped core bordered on the sides and bottom by suitable cladding material. The cladding material is chosen to have a refractive index lower than that of the core thus forming a channel shaped lightguide. In most configurations an upper cladding layer is included.
A structure that has received much interest recently is patterned after the typical optical fiber structure, i.e. a doped silica core with a silica cladding, and is fabricated using techniques developed in optical fiber technology, e.g. flame hydrolysis deposition (FHD). The preferred platform or substrate for these waveguides is a silicon wafer. The compatibility between the materials of the optical fiber and the planar integrated circuit allow common processing approaches. Also, more efficient and more reliable coupling between the optical fiber input/output can be obtained with this combination due to matching of thermo-mechanical, chemical, and optical properties.
The primary goal in photonic integrated circuit technology is the same as for semiconductor technology, i.e. packing multiple elements or devices into a small space. Routing of electrical leads in semiconductor devices is relatively unconstrained since electrical runners can be made with sharp bends. In photonic integrated circuits, the allowable bending radius is an important issue because it influences strongly the possible packing density of components. In general, given a certain waveguide index step or gradient, the loss introduced in the photonic circuit by the bent portion of a waveguide increases with decreasing curvature radius. Therefore, given a certain waveguide index step or gradient, the bending radius of a waveguide is limited to a value which gives losses acceptable in the economy of the circuit. For a given loss "budget" the potential packing density of components increases if the waveguide index step or gradient increases.
In single mode photonic integrated circuits, the allowable bending radius is sensitive to the index gradient between the core and the cladding materials, and also to the size of the waveguide. In high refractive index delta waveguides the optimum core size for a single mode waveguide is significantly smaller than the core size of a typical optical fiber. This difference in core size has important implications in coupling efficiency between the core of the planar integrated circuit waveguide and the core of the input/output fiber attached to the integrated circuit. The coupling loss between the fiber and the planar IC is minimized when the mode of the optical beam is preserved, i.e. the fiber and the IC have matched optical modes. This is achieved for fiber and waveguide having the same index delta by matching the core size. Consequently, to minimize insertion loss, the generally preferred objective is to have matching core sizes. However, this appears incompatible with the other important objectives of retaining the standard core size for the optical fiber, and using a smaller core size for the planar integrated circuit.
To overcome the core size difference it has been proposed to fabricate a transitional waveguide section in the region of the planar integrated circuit at the coupling between the planar waveguide and the fiber. In this transitional section the core of the planar waveguide is tapered so that the cross section of the core of the waveguide mates closely with the cross section of the core of the fiber. Beyond the tapered section, in the direction of the planar integrated circuit, the cross section area of the core of the waveguide is reduced to satisfy the optimum dimensions for planar waveguide cores.
A planar waveguide structure designed to implement this concept is described in Journal of Lightwave Technology, Vol. 10, No. 5, pp. 587-591, May 1992. In this paper a technique is proposed to taper the core of the planar waveguide using flame hydrolysis deposition (FHD). After deposition of the cladding layer on the substrate in the normal way, the core is created by scanning the substrate with the FHD flame while modulating the composition of the gases being fed to the torch, thus changing the composition of the deposited material in the scanning direction. Either the torch or the substrate can be physically moved, and the scanning direction is consistent in a single x-y-direction, i.e. it scans in only in the direction parallel to the waveguide being tapered. In theory, one could scan in the other direction, i.e. normal to the waveguide, but that would require more control than is allowed by the usual dynamic range of the process. In the manner described, longitudinal composition gradients, and corresponding index gradients, can be used to form the desired taper in the x-y plane of the planar waveguide, i.e. to taper the width of the waveguide. In a similar manner, but using a control program based on point-to-point layer composition, control of the composition in three dimensions can be achieved and the effective cross section of the waveguide can be tapered in both the x-y plane and the z-(thickness) direction. The capability of tapering in the z-direction is the desirable feature of this process since tapering in the x-y plane can easily be achieved using a conventional mask. However, in practice the technique is limited to producing tapers in one x-y direction only, due to the relatively limited dynamic range over distance of the compositional changes or the deposition mass flow rate that are obtainable with the technique. Moreover, the goal of tapering in the z-direction is achieved in this technique only with substantial process complexity and cost. A simpler and more cost effective method for tapering planar waveguides would be desirable, especially one which has no constraints on the direction(s) of the taper in the x-y plane, and which can produce relatively abrupt tapers at any position(s) in the photonic IC.